Superficial zone protein (SZP) is an abundant glycoprotein in human synovial fluid with lubricating properties

Superficial zone protein (SZP) is a 345 kDa glycoprotein which contains chondroitin sulfate and keratan sulfate chains [1]. The molecule is synthesized by the superficial zone chondrocytes of articular cartilage, but not by middle and deep zone chondrocytes. It is also made by synovial cells and is present in synovial fluid. SZP shows a high degree of homology with megakaryocyte stimulating factor (MSF) [2], camptodactyly-arthropathy-coxa vara-pericarditis protein (CACP) [3] and lubricin [4].